Acetyl-CoA: Although acetyl-CoA is mainly oxidized in the Krebs cycle or convened to fatty acids or ketone bodies, a varying percentage can be used as a substrate in co-translational, post-translational, or chemical modification acetylation. The covalent attachment of acetyl groups to biological substances is a common chemical modification in biological systems (Tsunasawa, S., et al., Methods Enzymol. 106:165-170 (1984); Driessen, H. P. C., et al., CRC Crit. Rev. Biochem. 18:281-325 (1985); Klein, U., et al., Proc. Natl. Acad. Sci. U.S.A. 75:5185-5189 ( 1978); Roskoski, R., Jr., J. Biol. Chem. 249:2156-2159 (1974); Jencks, W. P, et al., J. Biol. Chem. 247:3756-3760 (1972); Deguchi, T., et al., J. Biol. Chem. 263:7528-7533 (1988)). Acetylation is mediated by acetyltransferases, which catalyze the transfer of acetyl groups from acetyl coenzyme A to the --NH.sub.2 groups or --OH groups of biological molecules.
The concentration of acetyl-CoA in cells is regulated by its rate of synthesis and its rate of utilization and degradation. Acetyl-CoA is primarily synthesized from pyruvate generated from carbohydrates and amino acids (Ala, Thr, Gly, Ser, and Cys), from acetoacetyl-CoA generated from other amino acids (Phe, Tyr, Leu, Lys, and Trp), and from the .beta.-oxidation of fatty acids. A minor amount is synthesized by acetyl-CoA synthetase (Frenkel, E. P. et al., J. Biol. Chem. 252:504-507 (1977)). Acetyl-CoA may be used in the Krebs cycle or converted to fatty acids or ketone bodies. The acetyltransferase-catalyzed acetylation of proteins and peptides (Tsunasawa, S. et al., Methods Enzymol. 106:165-170 (1984); Driessen, H. P. C. et al., CRC Crit. Rev. Biochem. 18:281-325 (1985); Persson, B. et al., Eur. J. Biochem. 152:523-527 (1985); Augen, J. et al., Trends Biochem. Sci. 11:494-497 (1986); Allfrey, V. G. et al., Methods Enzymol. 107:224-240 (1984); Rudman, D. et al., J. Biol. Chem. 254:10102-10108 (1979)), as well as of biological substances other than proteins (e.g., glucosamine, choline, arylamine, arylalkylamine) (Klein, U. et al., Proc. Natl. Acad. Sci. USA 75:5185-5189 (1978); Roskoski, R., Jr., J. Biol. Chem. 249:2156-2159 (1974); Jencks, W. P. et al., J. Biol. Chem. 247:3756-3760 (1972); Weber, W. W. et al., Pharmacol. Rev. 37:25-79 (1985)) accounts for additional usage of endogenous acetyl-CoA.
Acetyl-CoA Hydrolase: Acetyl-CoA hydrolase (EC 3.1.2.1) hydrolyzes acetyl-CoA to acetate and CoA. This enzyme was first identified in pig heart in 1952 (Gergely, J., et al., J. Biol. Chem. 263:313-319 (1952)) and has subsequently been found in many mammalian tissues (Knowles, S. E., et al., Biochem. J. 142:401-411 (1974); Robinson, J. B., et al., Biochem. Biophys. Res. Commun. 71:959-965 (1976); Bernson, V. M. S., Eur. J. Biochem. 67:403-410 (1976); Grigat, K. P., et al., Biochem. J. 177:71-79 (1979); Prass, R. L., et al., J. Biol. Chem. 255:5215-5223 (1980); Soling, H. D., et al., Eur. J. Biochem. 147:111-117 (1985)). In rat liver, two isoenzymes of acetyl-CoA hydrolase have been found. One is located in the matrix space of mitochondria (Soling, H. D., et al., Eur. J. Biochem. 147:111-117 (1985)) and the other in the cytoplasm (Prass, R. L., et al., J. Biol. Chem. 255:5215-5223 (1980)). Although only the rat liver cytoplasmic enzyme has been purified to homogeneity (Prass, R. L. et al., J. Biol. Chem. 255:5215-5223 (1980)), the rat brain mitochondrial enzyme has also been partially purified and characterized (Robinson, J. B. et al., Biochem. Biophys. Res. Commun. 71:959-965 (1976)). The soluble cytosolic acetyl-CoA hydrolase found in rat liver is cold labile, inhibited by 5'ADP and activated by 5'ATP (Prass, R. L., et al., J. Biol. Chem. 255:5215-5223 (1980)). In contrast, the mitochondrial acetyl-CoA hydrolase is not affected by cold temperature (4.degree. C.), ADP or ATP (Soling, H. D., et al., Eur. J. Biochem. 147:111-117 (1985)). CoASH is a strong product inhibitor of the cold-labile enzyme, but only a weak inhibitor of the mitochondrial enzyme (Soling, H. D., et al., Eur. J. Biochem. 14 7:111-117 (1985) ).
Although there have been many studies on the physicochemical properties of acetyl-CoA hydrolase, little is known about its biological functions. It has been proposed that the enzyme may play a role in maintaining cytosolic acetyl-CoA and CoASH concentrations for both fatty acid synthesis and oxidation (Prass, R. L. et al., J. Biol. Chem. 255:5215-5223 (1980)). Bernson (Eur. J. Biochem. 67:403-410 (1976)) has suggested that the enzyme in hamster brown adipose may be involved in the post-hibernation warming-up process. In addition, it has been suggested (Namboodiri, M. A. A. et al., J. Biol. Chem. 255:6032-6035 (1980)) that arylalkylamine N-acetyltransferase in the rat pineal gland plays a key role in maintaining the circadian rhythms associated with melatonin synthesis and that acetyl-CoA hydrolase may also play a role in this process.
Acetyl-CoA hydrolase has been found to be an inhibitor of purified rat brain pyruvate carboxylase (Mahan, D. E., et al., Biochem. J. 145:25-35 (1974)) and choline acetyltransferase (Severin, S. E., et al., Biokhimia 32: 125-131 (1967)). It is also suspected of being an inhibitor of the acetyltransferase found in pituitary homogenates (Glembotski, C. C., J. Biol. Chem. 257: 10501- 10509 (1982)) and in crude yeast lysates (Travis, G. H., et al., J. Biol. Chem. 259:14406-14412 (1984); Dixon, J. E., et al., Methods Enzymol. 106:170-179 (1984)). In addition, acetyl-CoA hydrolase has been found to inhibit purified rat brain pyruvate carboxylase (Mahan, D. E. et al., Biochem. J. 145:25-35 (1975)), choline acetyltransferase (Severin, S. E. et al., Biokhimia 32:125-131 (1967)), and [acyl-carrier-protein] acetyltransferase (Lowe, P. N. et al., Biochem. J. 250:789-796 (1988)).